Shadows play an important role in lighting and rendering; as Da Vinci said, ‘an object appears most in relief when it is between light and shadow ’, which is called penumbra. When the light source has an extended area, the shadow transitions gently from darkness to softness, and gives strong visual cues to the boundaries of the object. Physically correct soft edged shadows (“soft” shadows) are expensive to compute in computer graphics; for this reason, they are rarely used in production lighting in computer graphics such as animation. As a result, lighters doing computer aided animation spend much time manually tweaking lights to emulate the look of soft shadows.
There has been much work on shadow algorithms for computer graphics. There have been two main approaches to soft (soft edged) shadow computation. One set of solutions is object space based; several papers describe penumbra computation using wedges or blurred wedges. Most recently, there is known a soft shadow algorithm that uses a single ray and edge visibility computation to reconstruct the shadowed area. This algorithm is significantly faster than distributed ray tracing in many situations, but the visibility algorithm is still bound by geometric complexity, and the advantage over classic ray tracing degrades significantly as the number of edges in the scene increases. Since most production setups have very complex geometry such as depicting dense foliage and fur, geometric or object space methods are still not practical.
It is known that for single point light sources, a depth map generated from the light view may be used to compute the shadow factor of any point in the scene. The associated shadow maps may suffer from bias and abasing, but given high enough resolution and proper sampling and filtering, they can deliver an antialiased shadow look independent of geometric complexity. Recently there has been much work on hardware-assisted soft shadow computation. Most of that work focuses on real time performance (speed), whereas the goal of the present method is generating soft shadows of a quality similar to those of distributed ray tracing.
It is also known to use ray tracing layered depth images (LDI) for computing secondary rays in image based rendering; however, because the source of LDI is often range data captured from views other than those of the light sources, and this method was prone to light leaks.
It is also known to perform penumbra computation using quantized multilayer depth images (MDI); by sampling the quantized MDI using filtered deterministic ray marching (via correlation in the light samples among the shading surfaces). The combination reduces light leaks but also caused blockiness and banding in the shadow image. Also known is ray tracing of multi-view shadow maps for creating soft shadows from area lights; and deep shadow maps for generating high quality shadows for fur and volume (from small light sources).